What do you mean, you can't take a crash dump? Here are some answers that people might come up with, and what I'd say in each case.
"I type CRASH A and hit ENTER, but then I get another crash message starting with fatal system error". Oh. Your system is so sick that the debugger can not accept commands. Furthermore, any attempts to use other debug commands like show calls, to determine the cause of this crash, are not going to work. Just reboot the computer and we will have to try later. Is this a reproducable symptom?
"It starts taking a dump, but then it finishes really quickly and it only uses one diskette". Oh. I'll bet you have more than 16 megs of memory on this machine. We had a bug in TSX some time ago that would prevent it from taking dumps on systems with over 16 megs. It only affected some of them. I don't really know when it was, but I'm sure it's fixed in 4.25 and on. If you don't want to bother upgrading, and you can reproduce this crash, you can probably get a dump by using TOTMEMPGS to force TSX to only use 16 megs. Now, that's gonna be painful to run with less memory, but just do it long enough to get us a crash dump and then set it back. You can follow this link if you need more instructions on TOTMEMPGS.
"Every time I press a key, I get another DB> prompt". Wow! I've seen this two times before. We never could figure out what it was about the computer. Wanna ship me the computer so I can try to figure it out? I thought not. Well, the only way to proceed is to use the debugger through the COM1 or COM2 port. That means that you have to reboot now -- no way to get a crash dump this time -- and configure your system for operating the debug terminal through the com port. I've put that topic on a separate page since we may refer to it elsewhere in this cave. Click here if you want to learn about how to set up the debugger on a COM port.